1. Field of the Invention
The invention relates to a microwave leakage field sensor for measuring moisture and/or density of dielectric materials.
2. Description of the Related Art
Both for application under conditions of a harsh industrial process and for serial measurement in the laboratory, microwave resonator technology affords the possibility of rapidly and precisely acquiring a statement about the moisture and density of the product to be investigated. What is essentially responsible for this is the particular method of separating the influence of moisture and density, which method can be employed with the use of resonators (EP 0 468 023).
In the resonator method, a standing microwave is generated in a suitable metallic cavity. An essential feature of this method consists in not just acquiring the no-loss effects of the interaction between the material moisture and the microwaves, that is to say all the property changes which are dominated by the real part of the dielectric constant. Rather the resonator method also enables the microwave power losses which influence the measurement result to be limited to the attenuation in the product (conversion into heat) and hence corruption due to other losses (leakage losses at surfaces or at coarse-grained scattering centers in the product, radiation losses, etc.) to be avoided. If a moist dielectric material is introduced into the resonator, then the resonant frequency is shifted and the width of the resonant curve increases. Density and moisture can be determined from these changes, as is explained in the abovementioned document.
The method presupposes the use of a resonator which is closed to a greater or lesser extent and from which no microwave energy can escape. Specifically, if field energy is lost by radiation, then this results in a corruption of the line width and of the resonant frequency, so that density and moisture can no longer be determined.
The limits in practical application of cavity resonators are imposed due to the fact that the product to be measured must be put into the sample tube of the applicator. In the case of many products, however, that is only possible in laboratory measurement if laboratory personnel fill the product manually into the sample applicator. In process measurements, in the case of products with good flowing or pouring behavior it is possible for a measurement to be effected in a bypass by transferring product from the main stream with the aid of conveying elements, filling it into the measurement tube and then conveying it back into the main stream again.
It is therefore desirable for process measurement technology to develop, as an expansion, a measuring device which has all the advantages of the patent, mentioned in the introduction, of a density-independent resonator measurement method for measuring moisture and, at the same time, can be directly incorporated in the main stream of the product flow without interfering with the process.
Although there are no difficulties at all in conducting microwaves from waveguides or cavities into the free space, so that the microwaves impinge on large-area plates, a stream of material in an industrial process, etc., in doing so an antenna-like radiation of microwave energy takes place, which, as already mentioned, effects the shifting of the resonant frequency and the widening of the resonant curve and thus corrupts the measurement result.
In the case of a previously known open leakage field resonator, the standing wave is generated by a coaxial line which is open on one side, resonator frequency and quality factor of the resonator being altered by a dielectric in the leakage field (Mesures Regulation Automatisme., Vol. 50, No. 1, January 1985, Paris FR, pages 67-70, XP002057631). The essential deficiency of this arrangement is that this form of leakage field has a distinct radiation behavior perpendicularly to the direction of the coaxial conductor. As a result, the essential precondition is not satisfied in order to be able to utilize the measurement of the losses and ot the resonant frequency shift for the purpose of separating moisture and material density. The layer thickness of the sample, the contact pressure between sensor and sample, the form of the sample, etc thus also have an interfering effect on the measurement signal.
A further previously known sensor with leakage field at the open line end of a coaxial conductor is intended to be used for measuring moisture (Thompson F: "Moisture Measurement Using Microwaves", Measurement and Control, Vol. 22, No. 7, September 1989, pages 210-215, XP000052731). In order to gain a certain degree of control over the problem of radiation and hence of the diverse interfering influences (form of the sample, etc.), the leakage field is limited to "a few millimeters penetration depth into the sample" This also does not allow reliable and accurate measurements for measuring moisture and/or density of samples of relatively large dimensions.